A few months ago my neighbor Mike and I decided to put my Big Green Egg through it's first real trial with a 12-hour brisket session. Mike is a stellar cook on all counts and I wanted to learn (steal) anything I could about grilling. As we were working the brisket through the hours we probed this large hunk of meat for temperature dozens of times, in dozens of locations (see the video timeline for proof). I've always had temperature probes and such, but Mike was using one of these high-end instant read digital thermometers and I realized this was a must for grilling. I decided to be patient and put a Thermapen on my wish list for Christmas. Now that I've used it a few times I feel like every grill owner should just get one of these with their grill. Why?
First, a large piece of meat is going to have several different temperatures throughout the cut. The Thermapen is fast enough that you can take several readings and get "x-ray vision" into the meat. Before you even slice it, you know where the cut will be more rare and more well.
Secondly, when you are grilling at really high temperatures the grill itself gets so hot that it's really hard to hold a probe in place for 10-20+ seconds waiting for a reading. Yes, you can put a big huge glove on but that takes a lot of time and results in fewer temperature checks. The Thermapen is so fast that you just probe it, read, and remove. 2-3 seconds tops.
If you love to grill, get a Thermapen. It gives you a lot more confidence on the grill. You can stop overcooking to be cautious. And since you can take a temperature so fast you can temp every single item on the grill before it comes off. Each and every cut can be just perfect.